Rastered or “flying” spot displays have recently shown promise as a solution for very small projector applications, such as cell phones, notebook computers, digital cameras, and the like. Such spot displays have their origin over one hundred years ago in mechanical televisions and phototelegraphy (i.e., facsimile). The principal behind early flying spot scanners used a bright, narrow beam of light which would shine through the holes of a Nipkow disk, which is a disk with a series of holes typically cut in a spiral pattern in the disk. This light would then illuminate the subject, usually standing in a darkened studio. The resulting scanning spot of light could usually complete sixteen or more scans per second. The light would then reflect back to one or more photoelectric cells. While television has long since abandoned this scanning method, some modern graphical scanners still use a flying spot scanning system.
Because of its simple approach and the improvement in laser technology, rastered or flying spot scanning and display, as noted above, are now being considered an attractive implementation for very small projectors. In modern application, median power, single mode lasers (usually around 100 mW) enable realistic design of such small projectors. The median power and single mode of these lasers allows for the laser mechanism to be less complex, which lends itself to such small applications. Such systems typically generate a picture by rasterizing a set of red, green, and blue laser beams across a screen while modulating the beam intensity.
One main drawback to rastered spot displays is speckling. Because of the coherence of laser light, with a narrow wavelength, direction range, and (often) polarization, light waves scattered by nearby points on the screen often interfere at the observer location (such as a human eye or camera). This interference generally introduces an amplitude or brightness noise in the perceived light, commonly described as “speckle,” which degrades the image quality. With ordinary light this speckle noise is averaged to near zero because each wavelength, incoming light angle, and two polarizations generally produces distinctly different speckle patterns. As all of these speckle patterns are uncorrelated, i.e. random in different ways, they average to zero as the number of them increases. Thus, one method that has been considered to alleviate some of the speckle noise would be changing the mode of the lasers. However, altering the single-mode nature of the laser would generally result in an unacceptable loss of display resolution.
Speckle effect can also be decreased by reducing one or more of the coherence degrees of light: polarization, temporal, and spatial. In general applications, a time averaging of a multiple speckle pattern is typically used, which results in the pattern appearing sequentially at the display screen. However, in the case of rastered or flying spot displays, time averaging cannot be used because the light beam is only present at any given spot on the screen for a very short period of time. Moreover, as noted above, the lasers should remain single mode spatially in order to preserve resolution.